Circuit arrangement for controlling a wiper motor

ABSTRACT

Suggested is a switching arrangement for actuating a wiper motor (10) for windshield wiper arrangements, which functions to protect the power switching element (13), designed as semiconductor, and the wiper motor (10) against permanent destruction caused by blocking and overload. The switching arrangement comprises a temperature monitoring of the semiconductor end stage (29), which short-circuits the power switching element (13) in case of a motor (10) overload and simultaneous heating up of the power switching element and shuts down the motor (10), as well as a parking position monitoring of the wiper lever, in order to shut down the motor completely if the wiper lever is blocked (FIG. 1).

STATE OF THE TECHNOLOGY

The invention is based on a switching arrangement for a wiper motor forwindshield wiper arrangements with a time-dependent andtemperature-dependent load monitoring, wherein the wiper motor can beactuated via a power switching element, in connection with a means fordetecting the parking position of a wiper lever and a thereon dependentcontrol of the power switching element. A known switching arrangement ofthis type for a wiper motor (DE 28 52 676 IC2) combines a switchingstage dependent on the rotational speed and a switching stage dependenton the temperature. The switching stage dependent on the rotationalspeed compares the time intervals between the pulses depending on therotational speed with a reference value and, in this case, is configuredsuch that a switching element will trigger a switching signal forturning off the motor only if the rotational speed of the motor is veryslow or the motor is blocked. Once the malfunction has been corrected,the wiper motor operation is started up again automatically by turningthe switching element on and off periodically. The motor temperature isadditionally monitored to avoid a long shut-down is period, designed toprotect the motor, by sensing the voltage drop at the armature windingof the motor, and the switching element can thus be switched off via anadditional switching stage. This occurs through a regular switching of arelay to alternate between a voltage source and a constant currentsource. If the constant current source is connected through the relayand via the motor to a ground, then the motor temperature is determinedwith the aid of the armature winding resistance. No motor temperaturecan be determined if the voltage source is connected through the relayand via the motor to a ground.

If several pulses are triggered with each wiping cycle, one disadvantageof the pulse detection depending on the rotational speed is that areference value is necessary, which takes on varying values, dependingon the angle position for the wiper lever, thus requiring an involvedcircuit. A further disadvantage is that the temperature of the switchingarrangement itself is not monitored. Thus, if a semiconductor switch isused as power switching element, the power switching element is notprotected against a thermal overload and thus is not protected againstdestruction.

Another disadvantage lies in the fact that it is not possible tosimultaneously monitor the temperature and supply the motor withvoltage. Thus, the motor must be shut down each time for the temperaturemonitoring, which increases wear and tear of the motor and shortens itsservice life.

SUMMARY AND ADVANTAGES OF THE INVENTION

The present invention provides an improved circuit arrangement foractuating a wiper motor for windshield wiper arrangements with atime-dependent and temperature-dependent load monitoring, wherein thecircuit arrangement comprises: a wiper motor that can be actuated via asemiconductor power switching element that is controlled by a controlunit that is responsive to a signal from a means for detecting theparking position of a wiper lever and, produces a thereon dependentcontrol signal with time dependent monitoring of the power switchingelement; circuit means for monitoring a chip temperature of at least thesemiconductor chip of the power switching element and for shutting downthe power switching element when a threshold temperature is detected;and wherein, in order to protect the wiper motor against an overload andblocking, the control unit responds to a time-dependent load monitoringsignal provided by the means for indicating the parking position whenthe chip-temperature monitoring means responds, and shuts down the wipermotor completely.

The arrangement according to the invention, has the advantage that themotor as well as the power switching element are protected through achip-temperature monitoring against a thermal overload. It is thereforepossible to omit a cooling of the semiconductor end stage and to uselow-cost SMD (Surface Mounted Device) technology for this stage. Inaddition, the switching arrangement is less involved.

To be sure, a temperature monitoring for semiconductor power switches isknown from the magazine Elektor 4/93, page 41, where a temperaturesensor and a thyristor are additionally integrated on a chip. If thepower switching element heats up strongly, the temperature sensorignites the thyristor and thus blocks the power switching element. Thisdoes not provide an overload safety for the load connected to the powerswitching element. Thus, when using a weak motor, it is possible that inthe load circuit, the blocking current for the blocked motor would leadto an insufficient heating up of the semiconductor end stage, and thechip temperature monitoring would not respond. The motor would then beblocked permanently and would be damaged.

As a result of the combination of parking position monitoring and chiptemperature monitoring, the semiconductor end stage and the motor areequally protected, wherein the semiconductor end stage is suitable formotors with varying capacity.

It must also be mentioned here that the temperature-dependent and thetime-dependent monitoring occurs at the same time as the wipingoperation, meaning that a start-up or a shut-down of the motor is notnecessary.

Various advantageous modifications and improvements of the featuresaccording to the basic invention are disclosed. One particular advantageis the option of being able to use a self-blocking FET as powerswitching element, for which the gate is connected to a controldepending on the means for detecting the parking position and which hasa thyristor that is parallel-connected to the gate-source-path.

Another advantage results from the option of being able to use a cheaperFET with higher start-up resistance R_(DSon) since the danger of athermal overload of the end stage is not given in the blocking case.

BRIEF DESCRIPTION OF THE DRAWING

One exemplary embodiment of the invention is shown in the drawing and isexplained in further detail in the following description. FIG. 1 shows ablock diagram for actuating a rear wiper motor and FIG. 2 shows voltagediagrams of the circuit for various operational cases.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENT

The switching arrangement according to the invention comprises a rearwiper motor 10 as a load, which is connected on one side to a ground 11.The positive pole 12 of a battery voltage is switched to a n-channel FET13, via the rear wiper motor 10. In this case, the drain 14 is connectedto the positive pole 12 and the source 15 to the rear wiper motor 10.Source 15 and substrate 16 of the FET 13 are short-circuited. The FET 13is controlled with a positive voltage, which is fed to the gate 18 by acharge pump 17 that is known per se. The charge pump 17 and thus alsothe FET 13 are clocked by a control signal U₂₀ at the control output 19of a control unit 20. Alternatively, the charge pump 17 can alsocomprise an internal clock generator, so that the FET 13 is switched ononly via a static signal from the control unit 20. In this exemplaryembodiment, the control unit 20 has a microcontroller. A toggle switch21, which is supplied by an ignition lock switch 30, (terminal 15), isassigned to the control 20 and turns the control 20 on and off. Thecontrol 20 receives parking position signals U₂₂ from a means fordetecting the parking position, which for this embodiment is a parkingposition sensor 22 (e.g., a Hall sensor).

Parallel to the gate-source-path of the FET 13, a thyristor 23 isconnected such that its anode 24 is connected to the gate 18 and itscathode 25 to the source 15. At its control electrode 26, the thyristor23 is controlled by a temperature sensor 27, which is connectedheat-conducting to the FET 13, as is known per se. A current-limitingresistance 28 is installed between the charge pump 17 and the gate 18 orthe thyristor 23. The current-limiting resistance 28 could be omittedwith a correspondingly high internal resistance of the charge pump 17.The FET 13, the thyristor 23, the temperature sensor 27, the charge pump17 and the current-limiting resistance 28 form the semiconductor endstage 29 of the switching arrangement.

As can be seen in the voltage curves in FIG. 2, the switchingarrangement for actuating a wiper motor according to FIG. 1 operates asfollows:

The rear wiper motor 10 is switched with the aid of a power switch,which in this embodiment is a self-blocking n-channel FET 13. The rearwiper motor 10 is actuated at point in time t₁ with the voltage U₁₀, independence on the voltage U₁₈ that is present at the gate 18. If thereis no voltage U₁₈ at the gate 18, then the is FET 13 blocks and the loadcircuit is switched off. If the voltage U₁₈ exceeds the FET 13 thresholdvoltage at gate 18, the FET 13 becomes conductive and the batteryvoltage present between the positive pole 12 and the ground 11 dropsprimarily at the rear wiper motor 10. It means that the voltage U₁₈would drop again and become negative and the FET would block once more.In order to prevent this, the charge pump 17 is connected to gate 18.The charge pump 17 provides the necessary additional positive voltageU₁₈, preferably double the battery voltage, which leaves the FET 13 inthe conductive condition.

The FET 13 is controlled by the charge pump 17, which is clocked by thecontrol 20 or is clocked internally. For the undisturbed operationaccording to FIG. 2a, the wiping lever performs the wiping activitypreset by the control 20.

A means for detecting the parking position, which for the presentembodiment is a parking position sensor 22, supplies parking positionsignals U₂₂ to the control 20 to detect interferences in the normalwiping operation. Two different digital signals appear in this case,depending on whether the wiping lever is in the parking position or not.In the present embodiment, the signals can alternate between 0 Volt and5 Volt or between 0 Volt and the battery voltage. The 0 Volt signaloccurs with the parking position. The signal changes are relevant forthe regulation of the control.

For the exemplary case according to FIG. 2b, a malfunction occurs atpoint in time t₁, owing to the fact that the wiping lever is frozen ontothe rear windshield. The signal changes consequently do not occur andthe FET 13 is blocked, following a maximum time T for supplying currentthat is preset by the control 20, and the rear wiper motor 10 is turnedoff. The maximum time T for supplying current depends on the adjusted,maximum possible wiping cycle length. The exemplary value provided forthe maximum possible wiping cycle length for an outside temperature of-40° C. is 7 seconds.

Another example of a malfunction that can be detected by the parkingposition sensor 22 is the blocking of the wiping lever during the wipingoperation as a result of snow blowing onto the windshield or otherobstacles.

Malfunctions such as the continuous wiping on a dry windshield, meaningan overload of the rear wiper motor 10, are detected in accordance withFIG. 2c through the temperature monitoring. For this, the thyristor 23,which is controlled by the temperature sensor 27, is parallel-connectedto the gate-source-path 18-15 of the FET 13. During the uninterruptedoperation, the thyristor 23 is blocked. In case of an overload of therear wiper motor 10, the FET 13 heats up. Once the maximum permissiblesubstrate temperature of the FET 13 is reached at point in time t₃, thetemperature sensor 27 ignites the thyristor 23, which short-circuits thegate-source-path 18-15. As a result of the series connection of thecurrent limiting resistance 28 with the ignited thyristor 23, theresulting voltage drop at the thyristor 23 is so small that it fallsbelow the threshold voltage of the FET 13 and the load circuit is shutdown. The charge pump 17 then functions as voltage source against ground11. In this circuit, the current limiting resistance 28, the conductivethyristor 23 and the rear wiper motor 10 are connected in series.

The thyristor 23 remains conductive as long as the anode current of thethyristor 23 is higher than its holding current. In that case, the rearwiper motor 10 continues to receive current. However, as a result of thevoltage drop at the current limiting resistance 28, the drop in thevoltage U₁₀ at the rear wiper motor 10 is negligibly small, which leadsto a shutting down of the rear wiper motor 10 (point in time t₃). Thecurrent limiting resistance 28 takes over the function of a voltagedivider.

If the rear wiper motor 10 is shut down, the signal change from theparking position sensor 22 to the control 20 does not occur (point intime t>t₃) and following the maximum time interval T for a supply withcurrent, preset by the control 20, the voltage drop at the thyristor 23becomes so small--because the charge pump 17 is shut down by connectingthe ground 11 to the resistance 28--that the anode current falls belowthe holding current and the thyristor 23 blocks (point in time t₂ +T).This results in an additional action of the temperature monitoring stagethat is conceived as feedback in that a blocking through shutting downof the motor 10 is simulated when the chip temperature monitoringresponds. As a result of this, the parking position sensor 22 astime-dependent load monitor shuts down the wiper motor completely andfinally.

I claim:
 1. A circuit arrangement for actuating a wiper motor (10) forwindshield wiper arrangements with a time-dependent andtemperature-dependent load monitoring, said circuit arrangementcomprising: a wiper motor (10) that can be actuated via a semiconductorpower switching element (13) that is controlled by a control unit thatis responsive to a signal from a means (22) for detecting the parkingposition of a wiper lever and produces a thereon dependent controlsignal with time dependent monitoring of the power switching element(13); circuit means for monitoring a chip temperature of at least thesemiconductor chip of the power switching element and for shutting downthe power switching element (13) when a temperature above a giventemperature is detected; and wherein, in order to protect the wipermotor (10) against an overload and blocking, the control unit respondsto a time-dependent load monitoring signal provided by the means forindicating the parking position when the chip temperature monitoringmeans responds, and shuts down the wiper motor (10) completely.
 2. Aswitching arrangement according to claim 1, wherein the means (22)provided for detecting the parking position, is a parking positionsensor, a Hall sensor or a contact disk, the signals (U₂₂) of which arefed to the control unit (20).
 3. A switching arrangement according toclaim 1, wherein a self-blocking FET is provided as the power switchingelement (13), the gate (18) of which is connected to the control unit(20) that depends on the means (22) for detecting the parking position,and that the circuit means for monitoring the temperature includes athyristor (23) that is parallel-connected to a gate-source-path (18-15)of the FET.
 4. A switching arrangement according to claim 3, whereinonce a specific temperature is reached, a temperature sensor (27) thatis mounted on the semiconductor end stage (29) ignites the thyristor(23) and short-circuits the gate-source-path (18-15) of the power switch(13).
 5. A switching arrangement according to claim 4, wherein the gate(18) is connected via a charge pump (17) to the control unit (20) thattransmits a control signal (U₂₀ ) to the gate, which is amplified by thecharge pump (17) such that the power switch (13) remains controllable inthe conductive state as well.
 6. A switching arrangement according toclaim 5, wherein a capacitor, which can be charged and dischargedcyclically, a controller or a discrete circuit that clocks the chargingpump (17) can be used as the control unit (20).
 7. A switchingarrangement according to claim 5, wherein a signal change of th means(22) for detecting the parking position occurs, either when the wiperlever leaves its parking position or when it returns to the parkingposition, and if this signal change does not occur, it leads to ashut-down of the wiper motor (10) following a maximum time interval (T)during which current is supplied and which is predetermined by thecontrol unit.
 8. A switching arrangement according to claim 5, wherein acurrent-limiting resistance (28) is connected between the charge pump(17) and the power switching element (13) as an automatic protection forthe semiconductor end stage (29).
 9. A switching arrangement accordingto claim 3 wherein the gate (18) is connected via a charge pump (17) tothe control unit (20) that transmits a control signal (U₂₀) to the gate,which is amplified by the charge pump (17) such that the power switch(13) remains controllable in the conductive state as well.
 10. Aswitching arrangement according to claim 9, wherein a capacitor, whichcan be charged and discharged cyclically, a controller or a discretecircuit that clocks the charging pump (17) can be used as the controlunit (20).
 11. A switching arrangement according to claim 9, wherein asignal change of the means (22) for detecting the parking positionoccurs, either when the wiper lever changes its parking position or whenit returns to the parking position, and if this signal change does notoccur, it leads to a shut-down of the wiper motor (10) following amaximum time interval (T) during which current is supplied and which ispredetermined by the control unit.
 12. A switching arrangement accordingto claim 9, wherein a current-limiting resistance (28) is connectedbetween the charge pump (17) and the power switching element (13) as anautomatic protection for the semiconductor end stage (29).